1. Field of the Invention
The present invention relates to a multiwavelength actively mode locked external cavity semiconductor laser useful in fiber optic telecommunications systems and in other applications employing wavelength division multiplexing of high speed digital or analog optical signals and, more particularly, to a multiwavelength actively mode-locked external cavity semiconductor laser for the simultaneous reliable generation of multiple optical carrier signals for use in such systems.
2. Description of the Prior Art
Prior art multiwavelength optical signal processing and transmission systems rely upon optical sources to generate optical carriers for modulation of digital, or analog, data by a multiplicity of modulators. The modulated optical carriers propagate within substantially identical single transverse spatial modes occupying substantially the same position in a transmission medium as may be provided by single-mode waveguides, such as optical fibers or planar optical waveguides, or by free-space optics. Each modulated optical carrier can be separately distinguished from all others by means of an optical filter designed to pass, to one or more output ports, a given optical carrier wavelength while rejecting all others presented at its input port. Such a modulated optical carrier can be demodulated to convert information carried thereupon to electronic form. Systems employing multiple optical carriers distinguished by wavelength are designated as dense wavelength division multiplexing (DWDM) systems.
The optical sources employed in prior art DWDM systems are discrete laser source components, one for each wavelength. Reliability of these sources is assured by practices such as extended life-testing and thermal, electrical, and mechanical tests known to those skilled in the art, as exemplified by Bellcore TR-NWT-000468. Such laser sources are typically continuous wave, single frequency, single spatial mode diode lasers, conventionally of the distributed feedback (DFB) design, such that, e.g., a 40-wavelength system employs 40 separate DFB laser sources and 40 separate manufactured subsystems containing additional fiber optic, electronic, and electromechanical components and their interconnections including a single fiber optic output port to carry each modulated optical carrier for further combination with all others. Therefore, each such subsystem includes overhead in the form of additional components that are duplicated for each separate optical carrier wavelength.
It is the reliance upon single wavelength optical sources which requires prior art multiwavelength system architects to design an entire subsystem, not merely one source, for each optical carrier wavelength in prior art systems. The major sources of complexity and cost of a multiwavelength transmissions systems product are electronic, optical, and electromechanical components, their assembly, and related costs for assembly and manufacturing processes. These costs all accrue separately for each optical source. Therefore, single wavelength optical sources represent a serious disadvantage for the manufactured system because their use requires that the latter costs must be aggregated to the extent of the number of optical carrier wavelengths employed. Systems relying upon such aggregation are large in size, consume excessive power, and suffer from reliability concerns or require undesirably frequent maintenance associated with the use of a large number of optical source components.
The prior art contains examples of continuous wave (cw) optical source components capable of emitting multiple optical carriers simultaneously. In general, cw sources exhibit inherent shortcomings as compared to pulsed sources. A technique capable of generating multiple cw optical carriers simultaneously, put forth by Zah et al., requires a monolithic chip containing multiple DFB lasers. For practical purposes, the complexity of such optoelectronic integrated circuits results in low yield and high cost. In addition, the difficulty of packaging and qualifying for reliability assurance, as required for telecommunication applications, a source module including 20 or more separate lasers, as these devices must, involves significant challenges over and above those of demonstrating the devices' functionality.
Unlike pulsed lasers, individual cw lasers, whether tunable or single-wavelength, are unable to emit multiple optical carriers simultaneously. In contrast to cw schemes, pulsed operation bestows a particular advantage with respect to the dynamics of net gain, defined as the difference between the roundtrip gain provided by an amplifying medium and round-trip propagation loss within the laser resonator expressed in dB. The net gain transiently exceeds threshold for the highest net gain mode by a margin sufficient to permit the net gain for a multiplicity of additional modes to exceed threshold and thus for them, too, to lase. Such a margin cannot be maintained in steady state cw operation within a single gain medium because there the steady-state phenomenon of net gain clamping ensures that only a single mode is able to reach threshold.
Mode-locked pulsed laser operation, by contrast, offers significant advantage over cw operation for multiwavelength operation, as shown by Delfyett et al. A conventional mode-locked pulsed laser operates with substantial equality between round-trip travel time of an optical signal within the laser resonator and the pulse period divided by an integer factor greater than or equal to unity. When substantially transform-limited, a mode-locked pulsed laser emits Gaussian pulses occupying a full-width half maximum frequency spectrum .DELTA..nu. determined by .DELTA.t.DELTA..nu.=0.4413, where .DELTA.t is the full-width half maximum pulse duration.
An actively mode-locked laser is one in which a periodic variation is impressed upon net gain. In an actively mode-locked semiconductor external cavity laser (AMSECL) of the type described by Delfyett et al., optical gain is provided by an angled-stripe semiconductor optical amplifier (SOA). The minimum fundamental period of pulses emitted by an AMSECL, i.e., for which the aforementioned integer factor is unity, is limited by the practical cavity size. For free-space optical components, an approximate practical lower bound would 167 psec for fundamental mode-locked pulses, corresponding to approximately a 2.5 cm cavity. In general, other considerations may indicate a design of substantially larger cavity AMSECLs. Either harmonic mode-locking in which the integer factor exceeds unity or pulse interleaving subsequent to the AMSECL can be employed to attain shorter periods than generated by the AMSECL, as taught in the prior art.
The AMSECL has a number of advantages over other mode-locked lasers. Because it is based on the angled-stripe SOA, it can be made small and relatively inexpensive. Only a single angled-stripe SOA is required as the optical gain element of the AMSECL. As a two-terminal device, the angled-stripe SOA is simple to design. It is similar to, but simpler in design than, a DFB laser. Additionally, there is no need to invoke the complexity involved in separate pumping and amplifying components as required for active fiber amplifiers. Beyond feedback and collimation optics, no additional components are required.
In a conventional AMSECL net optical gain in the angled-stripe SOA is varied by direct application of periodic electrical bias to the angled-stripe SOA at an RF frequency substantially equal to the laser pulsation rate. The frequency spectrum of pulses emitted by a conventional AMSECL is accordingly modified from that of a cw laser. Additional frequency components are emitted at discrete frequency intervals from the fundamental component, corresponding to the pulsation rate in the case of fundamental mode-locking. The number of substantial wavelength components associated with a given fundamental component is such that the total frequency spectrum occupied by a fundamental and associated additional wavelength components is substantially equal to the inverse of pulse duration. While mode-locked, the fundamental wavelength emitted by a laser is not uniquely distinguishable such that a uniform comb of frequencies is emitted as illustrated in FIG. 1, showing the case of a conventional mode-locked laser having a fundamental pulse repetition period of 2.5 GHz.
A prior art method seeks to exploit individual wavelength components within a mode-locked frequency comb as optical carriers, requiring pulse duration below 100 fsec to cover the 40 nm band required for multiwavelength systems. Typical prior art methods for generation of 100 fsec pulses require the use of active fiber amplifiers which are more expensive and complex than angled-stripe SOAs. Generation of 100 fsec pulses further requires more complex and expensive optical systems beyond those required by an AMSECL. Another disadvantage is the fundamental repetition rate of 100 GHz, as required to achieve 100 GHz channel spacing. For these reasons, the use of ultrashort pulses (below 1 psec) or individual mode-locked frequency components as optical carriers is not a preferred method for use in multiwavelength optical signal processing and transmission systems.
As illustrated in FIG. 1, conventional mode-locked lasers are characterized by one fundamental component and thus one single comb of frequency components. The comb is relatively narrow in frequency spectrum compared to channel spacings. Therefore, for the purposes of multiwavelength optical signal processing and transmission systems, it is preferred to employ the entire comb as a single optical carrier. Accordingly, such mode-locked lasers are considered as single wavelength mode-locked lasers from the standpoint of DWDM systems.
A multiple optical carrier source is desired which takes of advantage of the feature of pulsed lasers to reliably emit many wavelengths simultaneously.
A multiwavelength mode-locked (MWML) laser which addresses these problems has been proposed by Delfyett et al. In the proposed system, a MWML single stripe angled-stripe SOA laser is an AMSECL in which a multiplicity of fundamental frequency components is emitted, where each fundamental frequency component is associated with its own unique set of additional frequency components such that each fundamental frequency and additional frequency components make up a unique comb of frequency components such that, as illustrated in FIG. 2, a multiplicity of combs of frequency components is emitted by the MWML laser. In the time domain, the MWML laser emits pulses of overall duration approximately equal to the inverse of the spectral width of each comb in the frequency domain. Unlike other mode-locked lasers, the MWML laser simultaneously emits a multiplicity of pulses of differing wavelengths corresponding to the position of the combs in the frequency domain. An embodiment of this MWML laser is illustrated in FIG. 3.
FIG. 3 illustrates the Delfyett et al. MWML laser light source 10 comprising an angled-stripe SOA 12 which is biased with a periodic signal at a radio frequency from an RF source 14, and an intracavity spatial filter 16 which filters an output of the angled-stripe SOA 12 to define individual spectral components at a plurality of wavelengths. Circulating within the MWML cavity are pulses generated at the plurality of wavelengths simultaneously which have round trip optical cavity travel times and net gains which are substantially equal at each generated wavelength. The spatial filter 16 in conjunction with the grating 18 may be selectively tuned to predetermined wavelengths by translating the spatial filter 16 in a plane normal to the output of the spatial filter 16. The radio frequency signal is provided to the angled-stripe SOA 12 by clock source 14 in conjunction with a direct current bias so that the angled-stripe SOA 12 couples the cavity modes together to generate periodic mode-locked pulses. The angled-stripe SOA 12 preferably comprises an angled-stripe InGaAsP, an angled-stripe GaAs/AlGaAs or an angled-stripe SOA of another material composition with facet reflectivities of 10.sup.-6 or less. Such low reflectivity is necessary to keep the gain spectrum of the angled-stripe SOA 12 free of undulations due to Fabry-Perot modes which otherwise would interfere with the generation of multiple wavelengths, since some wavelengths would be emphasized while others would be muted by such undulations. Other bulk optics devices are also used in the design of FIG. 3, including lenses 20, 22, 24, and 26 and mirrors 28, 30, and 32.
The ability of the MWML laser light source 10 illustrated in FIG. 3 to simultaneously emit multiple frequency combs as illustrated in FIG. 2 is a unique characteristic which enables it to be used as a optical carrier source for multiwavelength optical signal processing and transmission systems. In such applications, each frequency comb is a discrete wavelength source which can independently serve as a carrier for optical signaling purposes. MWML laser light source 10 can thereby eliminate the need for a multiplicity of cw light sources to serve as carriers for a WDM transmission system, for each comb may be considered to represent a single wavelength source for WDM applications.
In practice, under modulation the linewidth of each of the frequencies within a comb may become broadened so as to merge each comb into a single spectral peak of width equivalent to the comb envelope, with each comb serving as a carrier wavelength for signaling purposes in WDM communications systems.
A MWML laser light source 10 of the type shown in FIG. 3 comprises a broad-spectrum gain medium such as angled-stripe SOA 12 and a cavity designed to provide simultaneous feedback at a multiplicity of wavelengths and to provide substantially identical round-trip travel time and net gain for pulses at each of the various wavelengths within the cavity. While the MWML laser source of FIG. 3 demonstrates the basic principle of multiwavelength AMSECLs (MW-AMSECLs), the design of FIG. 3 has limited potential for commercial application in that it cannot be readily placed in a hermetically sealed package containing no substantial organic materials such as is required for use in commercial telecommunications applications.
The present invention relates to alternative implementations of the MW-AMSECL of FIG. 3 which are better suited to broad commercial application in multiwavelength optical signal processing and transmission systems and which overcome the aforementioned limitations of the prior art.